Why are alkyl groups considered as 'electron-donating' groups according to MO theory? how can this effect be explained using an MO diagram and how does this explain why some groups are 'electron-withdrawing'?
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1$\begingroup$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. $\endgroup$– Community BotCommented Oct 6 at 22:35
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1$\begingroup$ I suppose that plain MO theory would be difficult to use in this sense, it is rather the concept of electronic density and electrostatic charge distribution that you might use to reason on this problem. To be fair, it is possible to obtain the density matrix for a set of MO that is solution of the HF (or any other higher level of theory) equations and from there you could probably build the electrostatic charge distribution and explain why and when groups are "electron withdrawing or not. $\endgroup$– Giuseppe BasileCommented Oct 6 at 23:15
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1$\begingroup$ On the other hand, there are some other theories that aim to use yet the charge density and its gradients to build the concept of functional groups from rigorous quantum chemical considerations, you might want to look for QTAIM (Quantum Theory of Atoms in Molecules) and topological approaches to the chemical bond $\endgroup$– Giuseppe BasileCommented Oct 6 at 23:16
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This response is certainly not the definitive one, as there isn’t a general method to classify and rationalize the effects of electron-withdrawing groups based on MOs. However, several relevant studies tackle this issue, each using a different approach to analyze molecular systems.
From a quick search, I found a few papers that might interest you. Keep in mind that the concepts you're exploring in terms of MOs are often discussed in the context of density matrices and electrostatic density isosurfaces.
Upon a quick research, I got a few results you might want to read. Bear in mind though that the concepts you are trying to grasp in terms of MOs are more likely treated in the literature in terms of density functional theories, and in this sense i would suggest:
- Reed, A. E., & Weinhold, F. (1985). Natural Population Analysis. Journal of Chemical Physics, 83(2), 735-746. DOI URL: https://doi.org/10.1063/1.449486
- Bickelhaupt, F. M., & Baerends, E. J. (2000). Kohn-Sham Density Functional Theory: Predicting and Understanding Chemistry. Reviews in Computational Chemistry, 15, 1-86. DOI URL: https://doi.org/10.1002/9780470125922.ch1
- Parr, R. G., Szentpaly, L. V., & Liu, S. (1999). Electrophilicity Index. Journal of the American Chemical Society, 121(9), 1922-1924. DOI URL: https://doi.org/10.1021/ja983494x
The last one talks about the electrophilicity index but it might still be worth looking up.
In terms of pure MO theory I think it would be possible to define, at least at the HF level which is quite poor for most applications, some sort of localized molecular orbitals which contain most of the informations on certain functional groups, but I am no expert and I do not have any specific suggestion for references.
